The present invention relates to an ion-conductive solid polymer-forming composition having high ionic conductivity, high tackiness and excellent shape retention, and more particularly to an ion-conductive solid polymer electrolyte endowed with these same properties which is highly suitable for use in applications such as secondary cells.
Electrolytes used in electrochemical devices such as secondary cells have until now been primarily low-molecular-weight substances that are liquid at or above room temperature, such as water, ethylene carbonate, propylene carbonate, and tetrahydrofuran. In lithium-type cells in particular, use is commonly made of low-molecular-weight organic liquid electrolytes which tend to evaporate, ignite and burn rather easily. To ensure long-term stability, a metal can must be used as the outer cell enclosure so as to increase the airtightness of the container. The result has been a considerable rise in the weight of electrical and electronic components which use low-molecular-weight organic liquid electrolytes, and a complex production process.
By contrast, the use of a polymer as the electrolyte allows electrolytes to be obtained which have a very low volatility and are not prone to evaporation. Moreover, such xe2x80x9cpolymer electrolytesxe2x80x9d that have a sufficiently high molecular weight can even be used as solid electrolytes which are not fluid at or above room temperature. They offer the dual advantage of serving as a solvent for ion-conductive salts and of solidifying the electrolyte.
As an example of this type of polymer electrolyte, in 1978, Armond et al. at l""Universitxc3xa9 de Grenoble in France discovered that lithium perchlorate dissolves in solid polyethylene oxide, and reported that when 1 M lithium salt is dissolved in polyethylene oxide having a molecular weight of about 2,000, the resulting complex exhibits an ionic conductivity at room temperature of about 10xe2x88x927 S/cm. Another group reported that when 1 M lithium salt is dissolved in polyethylene oxide having a molecular weight of about 200, which is liquid at room temperature, the ionic conductivity at room temperature is about 10xe2x88x924 to 10xe2x88x925 S/cm. Thus, it is known that polymeric substances such as polyethylene oxide with the ability to dissolve ion-conductive salts function as electrolytes.
Since then, similar research has been carried out on a broad range of largely polyethylene oxide-related polymers, such as polypropylene oxide, polyethyleneimine, polyurethane and polyester.
As noted above, polyethylene oxide, the most thoroughly investigated of these polymers, has a good ability to dissolve ion-conductive salts. However, because it is a semi-crystalline polymer, when a large amount of metallic salt is dissolved therein, the salt forms a pseudo-crosslinked structure between the polymer chains that leads to crystallization of the polymer. As a result, the actual ionic conductivity is much lower than the predicted value.
This is because an ion conductor dissolved in a linear polyether-based polymer matrix such as polyethylene oxide migrates with the local movement of polymer chain segments within amorphous regions of the polymer matrix. With the formation of a pseudo-crosslinked structure, the cations which carry the ionic conductivity are strongly coordinated by the polymer chains, greatly reducing cation mobility and thus lowering the conductivity. Such local movement of the polymer chains is referred to as Brawnian motion.
For this reason, a linear polyether-based polymer such as polyethylene oxide is a poor choice as the matrix polymer for an ion-conductive polymer electrolyte.
In fact, according to the literature to date, ion-conductive polymer electrolytes composed entirely of linear polymers such as polyethylene oxide, polypropylene oxide and polyethyleneimine generally have an ionic conductivity at room temperature of about 10xe2x88x927 S/cm, and at best not higher than about 10xe2x88x926 S/cm.
To obtain ion-conductive polymer electrolytes having a high conductivity such as this, a molecule must be designed which allows the existence within the matrix polymer of many amorphous regions of good ion conductor mobility, and which does not crystallize even when an ion-conductive salt is dissolved therein to a high concentration.
One such method is the attempt, described by N. Ogata et al. (Sen""i Gakkaishi, pp. 52-57, 1990), to introduce a branched structure into polyethylene oxide. Their work demonstrates that polyethylene oxide derivative-based ion-conductive solid polymer electrolytes having a high ionic conductivity (about 10xe2x88x924 S/cm at room temperature) can indeed be synthesized. However, such polymer electrolytes have not been commercialized on account of the sheer complexity of the polymer synthesis method involved.
The inventor previously disclosed that polymer electrolyte-forming polymers having a high ionic conductivity can be prepared by introducing polyoxyalkylene branched chains onto a natural polymeric substance such as cellulose and capping the terminal hydroxyl groups with suitable substituents, and that such polymers can be used to form solid polymer electrolytes having an excellent strength and a high conductivity (JP-A 8-225626 and JP-A 9-50824).
However, solid polymer electrolytes in which polyoxyalkylene branched chains have been introduced onto a natural polymeric substance such as cellulose have two drawbacks: (1) because the molecular weight per polymer chain (backbone) unit is large, any further increase in the fraction of polyoxyalkylene segments, which is where ion-conductive salt dissolution and migration takes place, per unit weight of the natural polymeric substance is difficult to achieve; and (2) the tackiness tends to be somewhat poor.
The present invention was conceived in light of these circumstances. An object of the invention is to provide an ion-conductive solid polymer-forming composition and an ion-conductive solid polymer electrolyte which have a high ionic conductivity, a high tackiness, a semi-interpenetrating polymer network (semi-IPN) structure, and excellent shape retention.
Conducting extensive and repeated investigations in order to achieve these aims, the inventor has found that an effective way to raise the ionic conductivity within a polymer electrolyte-forming polymer is to increase the proportion per unit weight of the polymer electrolyte-forming polymer in which polyoxyalkylene segments capable of dissolving an ion-conductive salt are introduced onto the polymer.
That is, a typical example in which polyoxyalkylene branched chains are introduced onto a conventional natural polymeric substance such as cellulose might involve the introduction of a 10-mole unit length polyoxyethylene group per cellulose unit. In this case, the molecular weight of the cellulose recurring units (C6H10O5) is 162 and the molecular weight of the 10-mole polyoxyethylene groups ((CH2CH2O)10xe2x80x94H) is 441. Hence, the fraction represented by the polyoxyethylene groups, which are the portions of the polymer that dissolve the ion-conductive salt, relative to the unit weight of the resulting cellulose derivative (polyoxyethylene fraction) is given by the ratio 441/(441+161)=0.733.
By contrast, if a polymeric compound such as polyvinyl alcohol (PVA), having a unit molecular weight lower than natural polymeric substances such as cellulose is used as the backbone, given that the molecular weight of the PVA recurring units (CH2CH(OH)) is 44 and the molecular weight of the 10-mole polyoxyethylene groups ((CH2CH2O)10xe2x80x94H) is 441, a higher polyoxyethylene fraction of 441/(441+44)=0.909 is achieved. The higher polyoxyethylene fraction enables a greater amount of ion-conductive salt to be dissolved, in addition to which the molecule has a larger number of polyoxyethylene segments where ion migration occurs, increasing ion mobility. The inventor has thus found that a high ionic conductivity can be attained in this way.
Also, when a film-type cell, for example, is assembled using a solid polymer electrolyte, in order for the solid polymer electrolyte to additionally serve as the binder component for the cell, the electrolyte must have both a high ionic conductivity, and the ability to bind the powdery battery active material. That is, it must be tacky. Moreover, film-type batteries made with solid polymer electrolytes generally have a positive electrode/solid electrolyte/negative electrode construction. Unlike cylindrical batteries in which this positive electrode/solid electrolyte/negative electrode composite is coiled and placed in a can, the absence of a coiling pressure in film-type batteries means that pressure is not applied between the positive electrode and the solid electrolyte and between the solid electrolyte and the negative electrode, allowing the solid electrolyte to separate readily from the positive electrode and the negative electrode. For this reason as well, the solid electrolyte placed between the positive electrode and the negative electrode, in addition to serving as an electrolyte, must also have the ability to strongly bond the positive and negative electrodes. In other words, it must have tackiness.
Pursuing the investigations even further based on the above, the inventor has additionally found that ion-conductive solid polymer-forming compositions comprising (A) a polymeric compound containing polyvinyl alcohol units of general formula (1) below and having an average degree of polymerization of at least 20, in which compound some or all of the hydroxyl groups on the polyvinyl alcohol units are substituted with oxyalkylene-containing groups to an average molar substitution of at least 0.3, (B) an ion-conductive salt and (C) a compound having crosslinkable functional groups have the ability to dissolve a large amount of ion-conductive salt because of their high oxyalkylene fraction. Moreover, the presence in the polymeric compound molecule of more oxyalkylene segments over which the ions can migrate increases ion mobility, enabling a high ionic conductivity to be achieved. In addition, such compositions have a high tackiness which allows them to function well as a binder component that firmly bonds the positive and negative electrodes. The inventor has discovered also that ion-conductive solid polymer electrolytes which have a semi-interpenetrating polymer network structure wherein molecular chains on the polymeric compound (A) are entangled with the three-dimensional network structure of a polymer obtained by crosslinking the compound (C) having crosslinkable functional groups and which contain the ion-conductive salt (B) can be prepared from such compositions. These polymer electrolytes have a dramatically improved shape retention. Moreover, because the matrix is amorphous rather than crystalline, such polymer electrolytes are endowed with a high ionic conductivity and a high tackiness, making them ideal for use as ion-conductive solid polymer electrolytes in film-type batteries and related applications.
Accordingly, the present invention provides:
(1) an ion-conductive solid polymer-forming composition comprising (A) a polymeric compound comprising polyvinyl alcohol units of general formula (1): 
wherein n is a number of at least 20, and having an average degree of polymerization of at least 20, wherein some or all of the hydroxyl groups on the polyvinyl alcohol units are substituted with oxyalkylene-containing groups to an average molar substitution of at least 0.3; (B) an ion-conductive salt; and (C) a compound having crosslinkable functional groups;
(2) an ion-conductive solid polymer electrolyte, which polymer electrolyte has a semi-interpenetrating polymer network structure in which molecular chains on the polymeric compound (A) are entangled with the three-dimensional network structure of a polymer obtained by crosslinking the compound (C) having crosslinkable functional groups, and contains the ion-conductive salt (B);.
(3) a binder resin comprising:
(A) a polymeric compound comprising polyvinyl alcohol units of the above general formula (1) and having an average degree of polymerization of at least 20, wherein some or all of the hydroxyl group on the polyvinyl alcohol units are substituted with oxyalkylene-containing groups to an average molar substitution of at least 0.3; and (C) a compound having crosslinkable functional groups; and
(4) a secondary cell comprising a positive electrode, a negative electrode and a solid polymer electrolyte layer, characterized in that the solid polymer electrode layer is composed of the above ion-conductive solid polymer electrolyte and lies between the positive electrode and the negative electrode.